The present invention relates generally to spindle ends employed in rotary tool drives and more particularly to a fatigue-resistant spindle end which is especially well suited for rotary-impact tool drives such as air, electric and hydraulic impact wrenches.
Spindle ends are used on portable air, electric and hydraulic tool drives for tools such as sockets. Although reference is made herein to sockets, it will be understood that other tools, such as rotary sanders, may be used in connection with the invention. A spindle end is generally in the shape of an elongated body which has a longitudinal axis of rotation, and comprises a means for engaging a rotary drive means (drive engaging means) at one end of the body, a male tool engaging tang at the other end of the body and a torque transfer section between the drive engaging means and the male tang. The torque transfer section and male tang are usually integrally formed and coaxially disposed about the axis of rotation. In a plane normal to the axis of rotation, the male tang has a substantially square cross-sectional configuration. The tang is adapted for being received in a substantially square female receptacle, of like dimensions, which is formed in the socket. A power source, such as an electric or air-driven motor supplies a rotational force or torque to the drive engaging means, which torque is transferred to the tang, and thereby to the socket, through the torque transfer section. The torque transfer section generally includes a cylindrical, journal portion which is received in a journal bearing so that the spindle end is axially supported but free to rotate.
The rotary drive means may apply a substantially constant rotational force or torque or an intermittent, high magnitude torque. In the latter case, the rotary drive means is generally referred to as a rotary-impact drive means and includes an impact mechanism. It will be understood by those skilled in the art that the drive engaging means is then constructed accordingly.
Spindle ends are generally designated by the nominal dimensions of the substantially square cross-sectional configuration of the male tang. Thus, a spindle end wherein the cross-sectional configuration has nominal dimensions of 1/2 inch, is generally referred to as a "half-inch drive" spindle end. Likewise, such a spindle end is used to engage a socket having a female receptacle of like dimensions, referred to as a "half-inch drive" socket. It will be understood that the recited dimensions are only nominal and that the female receptacle must be made slightly larger than the tang in order for the tang to be received therein.
Prior art spindle ends in portable power tools are subject to frequent fatigue failures. Fatigue is a phenomenon which leads to fracture in a load-bearing member under repeated or fluctuating stresses, even though those stresses may have a maximum value substantially less than the tensile strength of the member. Fatigue fractures generally initiate at some point of geometric discontinuity in the member. The discontinuity may be present due to the geometric design of the member, or as a result of wear or a manufacturing defect. Regardless of the source, the discontinuity serves to concentrate or locally raise the stress level in the load-bearing member. Thus, fatigue fractures are progressive in nature and generally begin as minute cracks at the point of discontinuity. The embryonic cracks grow incrementally under the action of cyclic stress and, upon reaching a critical size, failure of the loadbearing member can occur in a catastrophic manner.
The incidence of fatigue failures in spindle ends is particularly pronounced in those tools which have rotary-impact drives. Such tools are frequently used in applications where it is difficult to remove a fastener with constant torque means. For instance, air-driven impact wrenches are used to remove the lug nuts which secure automobile wheels. The lug nuts, and the studs associated therewith, are subjected to salt, moisture and other corrosive conditions. A rotary-impact-driven wrench can apply a large torque to the lug nut through the stored momentum of a rotating impact mechanism in the air wrench. The repeated application of this torque helps to overcome the restraining force which results from the products of corrosion that form on the lug nuts and studs.
A sudden, catastrophic failure of a spindle end in a rotary impact-driven wrench results in a very dangerous condition. Because the impact forces are high, the socket and broken portion of the spindle end can be propelled at high speed, which may cause injury to the operator or a bystander.
Prior art spindle ends have been constructed so as to include an abrupt change in geometry between the substantially square cross-sectional configuration of the male tang and the cylindrical configuration of the journal portion of the torque transfer section. The shoulder created by this abrupt change in geometry functions as a stress concentrator and frequently serves as the site for initiating a fatigue fracture. In the prior art, a fillet of specified radius has been employed to lessen the severity of the stress concentration due to the shoulder at the tang/journal portion juncture. Thus, design parameters for spindle ends in rotary tool drives have been promulgated by the American National Standard Institute ("ANSI") under ANSI specification B107.4. The ANSI specifications include the range of permissible radii of curvature for the shoulder fillets in spindle ends. It has been found, however, that adherence to the ANSI specifications still results in a substantial number of fatigue failures in spindle ends, particularly in applications involving impact wrenches.
Stress concentration may also occur as a result of engagement between the male tang and the tools received thereon. In prior art spindle ends, the shoulder at the tang/journal portion juncture of the spindle end also acts as a stop (in the axial direction) for the engagement of the tool on the tang. Because the female receptacle in the tool is necessarily slightly larger than the dimensions of the tang, the area of engagement between the spindle end and tool usually includes the shoulder fillet. Notches are caused by repeated incidences of such engagement and are exacerbated by the load between the tool and tang. In other words, because the tool engages the tang at the fillet, the load is borne primarily at the fillet and not evenly distributed over the surface of the tang.
Thus, a large load is carried directly at the area of stress concentration which is due to both the change in geometry at the shoulder of the spindle end and the notches formed therein.